1. Field of the Invention
The present invention relates to a slit exposure process camera for exposing a photosensitive material to make a printing plate, and more particularly to a mechanism of a slit exposure process camera for cutting a photosensitive material in a required length.
2. Description of the Prior Art
A slit scanning process camera is one of devices for reproducing an image of an original onto a photosensitive material to make a printing plate. The slit exposure process camera includes an exposing optical system for focusing a narrow optical image through a slit over the width of an original onto a photosensitive material. The whole image of the original is successively recorded onto the photosensitive material while the original is moved relative to the optical system and while the photosensitive material is conveyed in synchronism with the movement of the original.
The slit exposure process camera requires thorough flatness of the photosensitive material in the position of exposure and constant-speed conveyance of the photosensitive material to improve the quality of exposure. Some conventional types of the camera convey the photosensitive material while keeping it in sufficient flatness in the position of exposure. FIG. 1 illustrates a structure at the exposing position of one of those cameras. This camera includes an upstream roller pair UR and a downstream roller pair LR, which are respectively disposed upstream and downstream from the exposure position EP, respectively, to convey a photosensitive material PM. The rotating speed of the downstream roller pair LR is set to be slightly greater than that of the upstream roller pair UR, whereby the photosensitive material is pulled by the roller pairs with appropriate tension and is conveyed in straight and flat manner. A driving mechanism of the upstream roller pair is provided with a one-way clutch for preventing excessive tension to the photosensitive material PM due to difference in the rotating speeds. In a typical mechanism, the upstream and downstream roller pairs UR and LR are substantially the same, and a higher rotating speed of the downstream roller pair LR is attained by a greater outside diameter of the downstream rollers.
The photosensitive material PM is held in a stationary position after completion of exposure and cut at the rear end of the exposed area with a cutter CT. The exposed area of the photosensitive material cut in the above manner is sent to the subsequent process including development and fixation. Meanwhile, the photosensitive material PM before exposure is held in the cutting position with the upstream roller pair UR. With start of another exposure process, the unexposed photosensitive material is conveyed towards the downstream roller pair LR by rotation of the upstream roller pair UR. The free end of the photosensitive material PM passes through the exposure position EP, and it is then held with the rotating downstream roller pair LR. The photosensitive material PM is thus kept in sufficient flatness by appropriate tension between the pairs of rollers again.
In this type of device, however, a certain length of the photosensitive material PM from the free end is exposed while being held only by the upstream roller pair UR; thus the portion is exposed without tension between the two pairs of rollers. The part of the photosensitive material is cut off afterwards to be discarded because the image recorded on that area is not stable nor exact. The unstable and non-exact image reproduction is attributed to two factors: the first one is the irregular movement of the photosensitive material PM when its free end is inserted into the downstream roller pair LR, and the second one is curvature or distortion of the photosensitive material PM due to the unsupported free end.
The irregular movement of the photosensitive material PM is caused by the fact that the free end of the photosensitive material PM reaches the downstream roller pair LR to be stopped a moment there, and then moves again to be drawn forward. This movement distorts the flatness and the constant-speed conveyance of the photosensitive material PM.
The second cause of the unstable image reproduction, that is, the curvature near the free end of the photosensitive material PM, is inherent in the photosensitive material due to the fact that the photosensitive material PM before exposure is wound on a roll.